As an optical recording disk, which can record, reproduce, and erase information or data, an optical disk of phase change type having chalcogenide as its recording layer material is known (e.g. U.S. Pat. No. 3,823,386). Generally, crystallized state of the recording layer is treated as non-recording state, and, recording signals are attained by rapid heating and rapid cooling of the material to change it to amorphous state. Also, by heating to set back to crystallized state again, signals are erased. Recording layer, if formed by usual layer-forming method, is formed as amorphous state, and, accordingly, as far as the above-described recording system is adopted, recording layer is to be crystallized beforehand. Usually, this crystallization process is called initialization.
For the initialization, methods using various types of energies are known, as is shown in Patent Gazette of Japanese Patent Laid-open No. Sho 47-26897; for example, electric energy, radiation heat, light of photo flash lamp, laser beam energy, etc.
On the other hand, methods are proposed to make the initialization unnecessary, in which a recording layer of crystallized state is obtained by heating the substrate while forming the film.
As the structure of the optical disk, it is usual to provide a protection layer adjoining to the recording layer to protect the latter from the environment such as oxygen, water, and so on. Also, in order to improve repetition capability of recording and erasing, such structure as to disperse the recording material particles into the thermally stable protective material is proposed (e.g. U.S. Pat. No. 4,621,032, Patent Gazette of Japanese Patent Laid-open No. Sho 57-208648, and Sho 62-226438). The theory of these conventional technology is that, if the recording material particle in the thermally stable protection layer is melted by radiation of laser beam, constituent element of the recording material can move only within the limited space of "particle", and, therefore, if the particle size is sufficiently small, movement of the element of the recording material is not accumulated, or deterioration by the repetition of recording and erasing does not result. As the method to disperse recording material into the protection material, co-deposition of recording material and base material, or sputtering with plural sputter targets is proposed.
As the method of film-forming for each layer of the optical disk, electron beam deposition, sputtering, or CVD is well known, and these method can provide optical information recording medium having satisfactory characteristics for practical use, according to some reports. These, however, still had the following subjects.
For instance, in view of mass production, it is desirable that the layer forming of the recording layer and the initialization therefor are simultaneously carried out. However, if the recording layer is formed, while heating the substrate, to get the layer in crystallized state, it is necessary to expose the substrate at high temperature of more than 100-150 C., and resin plate of acrylic resin, polycarbonate or such can not be employed, due to its deforming. Referring to other methods, neither a method suitable to initialize the whole recording medium beforehand has been investigated, nor a method with high productivity has been found.
Referring to the idea to disperse the particles of recording material in the protection material, the present inventors tried to manufacture optical information recording medium in accordance with the method as publicized so far. They, too, however, could not get such enough improvement as expected, of repetition life of recording and erasing. This is because the constituent element of the storage medium is not dispersed as particles having desired composition and proper size, even if the elements could be dispersed into the base material. Thus, although the general idea of the structure of the optical information recording medium has been known, practical method of manufacturing the recording medium having such structure is not yet available.
As for the film-forming method for a thin layer, laser sputtering is known (Solid State Technology/December 1987 p.39-41). The laser sputtering, called also laser ablation or laser deposition, applies pulse laser of high energy density to a solid state target to form a layer on the opposite substrate. This method, unlike the usual sputtering, does not need plasma state, and has various favorable features, such as possible film formation in vacuum atmosphere. However, with relatively short history of development, laser sputtering is not yet established as reliable film-forming technology, and any attempt to apply this for the manufacture of optical information recording medium has not been reported.